The variations in the strength of a signal received at a ground station is most easily described in terms of the composite signal attenuation. This quantity is defined as the variations induced by vehicle spin and plasma effects in the signal radiated from the transmitting antenna as determined by measuring the signal power received on the ground. It is plotted in dB relative to the maximum free space value of the received signal. The slow variations of the received signal with range and antenna reflection losses due to plasma detuning are added as an attenuation bias to obtain the absolute attenuation.
The purpose of the simulator is to produce a signal variation that would be observed in flight. There are typical curves for composite signal attenuation. A high altitude curve (no plasma) can be calculated or measured. Others which represent plasma effects can be computed. The simulator of the present invention effectively duplicates the fast cyclical variations of the composite signal attenuation by using a properly programmed voltage controlled attenuator. The free space propagation losses and the transmitting antenna losses are accounted for with a fixed attenuator.
It is noted that this invention provides a simple way of determining the dynamic properties of a microwave link prior to an actual flight. Heretofore it was necessary to mount an actual payload or mockup on a turntable and driving it at a speed corrsponding to the spin rate of the vehicle. This mechanical system has severe limitations. It cannot be used for fast spinning vehicles and only a few orientations of the payload are possible because of mechanical constraints. In addition, variations due to plasma effects cannot be simulated. The invention solves all these problems.
This invention can be used prior to rocket test flights to determine the dynamic properties of a microwave link in order to discover any deficiencies before an acutal flight. It also gives the ground receiver operators an opportunity to observe "in-flight" signals before a launch.